The invention relates to a surface emitting semiconductor laser, its manufacturing method, and a manufacturing method of an electron device.
In recent years, attention has been paid to a VCSEL (Vertical Cavity Surface Emitting Laser) as a laser for optical communication. Such a surface emitting laser has a structure in which upper and lower surfaces of an active layer are sandwiched between an n-type reflective layer and a p-type reflective layer. Generally, a DBR (Distributed Bragg Reflector) made of a semiconductor multilayer film is used for each of the n-type reflective layer and the p-type reflective layer.
Generally, a structure called a post type mesa structure is used as a structure of the surface emitting laser (for example, refer to Patent Document 1: JP-A-2001-210908). FIG. 15 shows an example of the surface emitting laser having such a post type mesa structure. As shown in FIG. 15, to manufacture such a surface emitting laser, an n-type DBR layer 102, a lower clad layer 103, an active layer 104, an upper clad layer 105, a p-type AlAs layer 106, a p-type DBR layer 107, and a p-type GaAs contact layer 108 are sequentially grown on an n-type GaAs substrate 101. After that, the lower clad layer 103, active layer 104, upper clad layer 105, p-type AlAs layer 106, p-type DBR layer 107, and p-type GaAs contact layer 108 are anisotropically etched in the direction perpendicular to the substrate surface by lithography and dry etching, thereby working into a cylindrical mesa shape having a diameter of about 30 μm. Subsequently, by selectively oxidizing the p-type AlAs layer 106 from an outer periphery, an oxide layer 109 is formed in a ring shape. A current constriction structure to inject a current into the active layer 104 at high efficiency is formed. Subsequently, a ring-shaped p-side electrode 110 is formed in the mesa upper portion. An n-side electrode 111 is formed on the back surface of the n-type GaAs substrate 101. An opening portion of the p-side electrode 110 becomes a light outgoing window. In the surface emitting laser, by applying a voltage across the p-side electrode 110 and the n-side electrode 111 and injecting a current into the active layer 104, a laser resonance occurs between the n-type DBR layer 102 and the p-type DBR layer 107 and a laser beam is taken out in the direction perpendicular to the substrate surface. In the surface emitting laser, if the current constriction diameter (diameter of the p-type AlAs layer 106) is equal to about 15 μmφ, the current is uniformly and efficiently injected into the active layer 104 just under the p-type AlAs layer 106, so that the efficient laser oscillation can be performed.
In such a surface emitting laser, control of a lateral mode for oscillating is an important problem. Ordinarily, it has been known that an FFP (Far Field Pattern) of the surface emitting laser which has been put into practical use is liable to become a multi-mode pattern having two or more peaks. On the other hand, in order to couple a laser beam with a fiber through a lens upon applying to communication or the like, a beam in a Gaussian distribution shape of a single peak (generally, beam of a single lateral mode) is still necessary from a viewpoint of an optical design.
As shown in FIG. 16, however, in the surface emitting laser of the post type mesa structure, particularly, in the case of selectively oscillating the single lateral mode of the single peak, a method of decreasing the current constriction diameter of the p-type AlAs layer 106 is generally used. That is, since a size of constriction diameter is almost proportional to the number of modes to be excited, if a current injecting region is narrowed to a certain degree, naturally, the single mode which is excited in a narrow area can be obtained as output light. However, if such a method is used, since it is necessary to extremely decrease the constriction diameter to 4 μmφ or less (refer to Non-Patent Document 1: M. Grabherr et al., “IEEE. Photon. Tech. Lett”, Vol. 9, No. 10, page 1304, or the like), an allowable width is narrow to uniformly manufacture the current constriction structure with high controllability on a wafer and a manufacturing yield of the surface emitting laser deteriorates extremely. Since the current is concentratedly injected to the region of the p-type AlAs layer 106 whose area is smaller than that of the ordinary surface emitting laser by one digit and such a narrow region, a device resistance is high (normally, 100Ω or more), an output is small, and efficiency is also low. High-frequency driving which is inherently necessary for communication is also difficult as a result of impedance mismatching. Since single mode performance has output dependency, there is a deterioration tendency (mode competition, or the like) at the time of the high-frequency driving, or the like, it is considered to be difficult to apply such a surface emitting laser to light transmission of the high-frequency driving.
In Patent Document 2 (JP-A-9-246660), there has been proposed a surface emitting semiconductor laser apparatus constructed in such a manner that an n-type GaAlAs optical guide layer, an active layer, and a p-type GaAlAs optical guide layer are sequentially grown on an n-type GaAs substrate, a circular diffraction grating is formed on the p-type GaAlAs optical guide layer, an n-type GaAlAs current block layer is grown on the diffraction grating, a window is formed in the n-type GaAlAs current block layer, and a p-type GaAlAs optical guide layer is further grown on the window. However, according to Patent Document 2, since the diffraction grating is formed by the lithography and the etching, such a technique differs from the present invention in which the refractive index distribution is formed in the outgoing window portion by oxidization.